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Most people are familiar with mail headers such as From:, To:, Subject: etc. but there are many more which most people never see. Quite often we need to see the full internet mail headers from an e-mail that is causing problems. This article explains how to get hold of them in some of the more common e-mail clients.
On the main menu bar, go to View → Headers → All
and then select the relevant e-mail message. You sHould see the full internet headers in place of the usual display of Subject, From, Date, To.